Water absorbent material and absorbent article using same

ABSTRACT

The water absorbent material of the present invention is composed of a copolymer of an anhydropolyamino acid having at least one ethylenically unsaturated double bond in a molecule, a water-soluble monomer having an ethylenically unsaturated double bond and polysaccharides, and has high water absorption ratio and high water absorption rate in pure water or water having a low ion content and also has high absorption properties for high concentration salt-containing solutions.

TECHNICAL FIELD

The present invention relates to a novel and useful water absorbentmaterial and an absorbent article which utilizes such a material. Inparticular, the present invention relates to a water absorbent materialand an absorbent article for use in the absorption of solutionscontaining a high concentration of salts, such as sea water, calciumchloride deliquescent solutions, blood, and other body fluids (such asurine and sweat).

A water absorbent material of the present invention can be widely usedin fields such as sanitary products and household articles such asdisposable diapers, pads, and sanitary napkins, water sealing materials,soil conditioners, anti-condensation coatings, water-storing materialsfor use in agriculture and horticulture, and water swelling rubbers.

BACKGROUND ART

Conventional water absorbent materials include hydrolysates of graftpolymers of starch and acrylonitrile, and partially neutralized productsof crosslinked polyacrylic acid. Furthermore, examples of waterabsorbent polyamino acid based resins obtained by hydrolyzing apartially crosslinked product of polyamine polyaspartate have also beendisclosed in Japanese Unexamined Patent Application, First Publication,No. Hei 7-309943 and Japanese Unexamined Patent Application, FirstPublication, No. Hei 9-169840. However, water absorbent materialsproduced from polyamino acid based resins display insufficient gelstrength. Furthermore, although water absorbent materials produced fromacrylic resins are typically capable of absorbing between severalhundred and several thousand times their own weight of fresh water, thiswater absorbing ability decreases to an extremely low level for waterwhich contains salts. As a result, in the water absorbent materialdevelopment field, various trials for enhancing the water absorptioncapability for water containing salts have been made.

As the water absorbent material having an enhanced water absorptioncapability for water containing salts, for example, the followingsuggestions have been made wherein an ionic water absorbent materialhaving a small salt resistance is used in combination with a nonionicwater absorbent material having a large salt resistance.

(1) A water swelling polymer comprising a copolymer of an ethylenicallyunsaturated monomer with a carboxyl group and a base thereof, and apolyoxyalkylene glycol allyl ether with a hydrophobic group at oneterminal (Japanese Unexamined Patent Application, First Publication, No.Sho 62-27408); and (2) a water absorbent polymer comprising a copolymerof an ethylene based unsaturated monomer with a carboxyl group and anassociated base, and an alkylpolyoxyalkylene glycol mono(meth)acrylatewith an alkyl group at one terminal (Japanese Unexamined PatentApplication, First Publication, No. Hei 3-93815).

However, in the conventional water absorbent materials described above,although the important characteristics such as the water absorptioncapability (volume of water absorption, speed of water absorption) andthe salt tolerance are improved to some extent, the improvement is notalways sufficient.

Furthermore, in order to solve the problems described above, waterabsorbent materials comprising a copolymer of either a sulfoalkyl(meth)acrylate or an acrylamide (Japanese Unexamined Patent Application,First Publication, No. Hei 10-81714) or a copolymer of a nonionicmonomer and acrylic acid (Japanese Unexamined Patent Application, FirstPublication, No. Hei 9-143210) have also been proposed. However,although these water absorbent materials offer an improved waterabsorption of water which contains salts, the water absorption for purewater or water with only small amounts of ions actually decreases, andthe initial water absorption speed is also slow.

In addition, as an example of a water absorbent material comprising acombination of a polyamino acid and a copolymer comprising a polyacrylicacid, the water absorbing agent composition comprising a polyamino acidand a crosslinked polyacrylate polymer as the main constituents isdisclosed in Japanese Unexamined Patent Application, First Publication,No. Hei 7-310021. However, this water absorbent material exhibits littlewater absorption capability, and even when the surface of the waterabsorbent resin is crosslinked using a surface crosslinking agent, thegel strength displays no improvement.

An object of the present invention is to provide a water absorbentmaterial which displays superior water absorption of solutions with highconcentrations of salts such as sea water, calcium chloride deliquescentsolutions, blood, and other body fluids (such as urine and sweat) andfurther suffers no deterioration in the water absorption ratio or thewater absorption speed relative to pure water or water with a low ioncontent.

DISCLOSURE OF INVENTION

As a result of intensive investigations aimed at resolving the problemsdescribed above, the present inventors have found that a water absorbentmaterial comprising a copolymer of an anhydropolyamino acid having atleast one ethylenically unsaturated double bond within each molecule(A), and a water soluble monomer having an ethylenically unsaturateddouble bond (B) displayed a high level of water absorption capabilityrelative to water incorporating a high concentration of salts, and as aresult were able to complete the present invention.

In other words, the present invention provides a water absorbentmaterial comprising a copolymer of an anhydropolyamino acid having atleast one ethylenically unsaturated double bond within each molecule(A), and a water soluble monomer having an ethylenically unsaturateddouble bond (B).

The present invention also provides a water absorbent materialcomprising a copolymer of an anhydropolyamino acid having at least oneethylenically unsaturated double bond within each molecule (A), a watersoluble monomer having an ethylene based unsaturated double bond (B),and a polysaccharide (C).

In addition, the present invention also provides an absorbent articlecomprising a liquid-permeable sheet, a liquid-impermeable sheet, and anabsorber comprising a water absorbent material and a fiber materialarranged between the liquid-permeable sheet and the liquid-impermeablesheet, wherein the water absorbent material is a water absorbentmaterial comprising a copolymer of an anhydropolyamino acid having atleast one ethylenically unsaturated double bond in a molecule (A) and awater soluble monomer having an ethylenically unsaturated double bond(B).

In addition, the present invention also provides an absorbent articlecomprising a liquid-permeable sheet, a liquid-impermeable sheet, and anabsorber comprising a water absorbent material and a fiber materialarranged between the liquid-permeable sheet and the liquid-impermeablesheet, wherein the water absorbent material is a water absorbentmaterial comprising a copolymer of an anhydropolyamino acid having atleast one ethylenically unsaturated double bond in a molecule (A), awater soluble monomer having an ethylenically unsaturated double bond(B), and polysaccharides (C).

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description of the water absorbent material according to thepresent invention is as follows.

First, the water absorbent material comprising a copolymer of ananhydropolyamino acid having at least one ethylenically unsaturateddouble bond in a molecule (A) (hereinafter referred to as apolymerizable anhydropolyamino acid (A)) and a water soluble monomerhaving an ethylenically unsaturated double bond (B) (hereinafterreferred to as a water soluble polymerizable monomer (B)) will beexplained.

Examples of the polymerizable anhydropolyamino acid (A) include thoseprepared by reacting an anhydropolyamino acid having no ethylenicallyunsaturated double bond in a molecule (A-1) (hereinafter referred to asan anhydropolyamino acid (A-1)) with a compound which has anethylenically unsaturated double bond and a functional group havingreactivity with the anhydropolyamino acid in a molecule (A-2)(hereinafter referred to as a polymerizable compound (A-2)) or thoseprepared by the polycondensation reaction of maleic amhydride, fumaricanhydride or malic anhydride and ammonia with heating.

Examples of the anhydropolyamino acid (A-1) include anhydrides ofpolyaspartic acid and polyglutamic acid. Among these anhydropolyaminoacids, polysuccinimide as an anhydride of polyaspartic acid is preferredin view of industrial availability. These compounds may have a linearstructure or a branched structure.

In addition, a basic skeleton of the anhydropolyamino acid (A-1) maycontain a unit of an amino acid other than glutamic acid and asparticacid.

Examples of the unit of the amino acid other than glutamic acid andaspartic acid include units of aliphatic α-amino acid such as glycine,alanine, valine, leucine, isoleucine, serine, threonine, asparagine,glutamine, lysine, ornithine, cysteine, cystine, methionine, proline,hydroxyproline, or arginine; aromatic α-amino acid such as tyrosine,phenylalanine, tryptophan, or histidine; α-amino acid whose side chainfunctional group is substituted; aminocarboxylic acid such as β-alanineor γ-aminobutyric acid; dipeptide (dimer) such as glycyl-glycine oraspartyl-phenylalanine; and tripeptide (trimer) such as glutathione.These amino acids may be optically active substances (L-isomer,D-isomer) or racemic modifications. These amino acid units may exist inthe form of a random copolymer or a block copolymer after being combinedwith glutamic acid or aspartic acid.

There are no particular restrictions on the method of producing theaforementioned anhydropolyamino acid (A-1). Examples of suitableproduction methods include (1) heating D/L-aspartic acid and performinga dehydration condensation, (2) heating D/L-aspartic acid and performinga dehydration condensation in the presence of a catalyst such asphosphoric acid, (3) heating D/L-aspartic acid in a suitable solvent andin the presence of a catalyst such as phosphoric acid, and performing adehydration condensation, (4) heating and reacting maleic anhydride,fumaric acid or malic acid with ammonia, and forming theanhydropolyamino acid via a maleimide or a maleamic acid intermediate,and (5) heating and reacting maleic anhydride, fumaric acid or malicacid with ammonia to generate a maleimide or a maleamic acid, and thenproducing the anhydropolyamino acid by further reaction in the presenceof a catalyst such as phosphoric acid. A water absorbent material of thepresent invention can utilize an anhydropolyamino acid obtained from anyof the above methods.

There are no particular restrictions on the polymerizable compound(A-2), although from the viewpoint of reactivity, compounds representedby the general formula [I] shown below are preferred.

(wherein, R¹ represents at least one type of functional group selectedfrom a group consisting of an amino group, an epoxy group, a carboxylgroup, a carbodiimide group, an oxazoline group, an imino group and anisocyanate group, Q represents an alkylene group of 1 to 10 carbonatoms, and R² represents a hydrogen atom or an alkyl group of 1 to 4carbon atoms.)

Examples of compounds represented by the by the aforementioned generalformula [I] include glycidyl methacrylate, glycidyl acrylate, acrylicacid, methacrylic acid, 2-methacryloyloxyethyl isocyanate, and2-isocyanatomethyl acrylate.

Examples of methods of reacting the anhydropolyamino acid (A-1) and thepolymerizable compound (A-2) include (1) adding the polymerizablecompound (A-2) directly to a powdered sample of the anhydropolyaminoacid (A-1) and mixing; (2) dissolving the anhydropolyamino acid (A-1) inan aprotic organic solvent such as dimethyl formamide, dimethylacetamide, N-methyl pyrrolidone, N,N′-dimethyl imidazolinone, dimethylsulfoxide, or sulfolane, and then adding and mixing the polymerizablecompound (A-2); (3) dispersing the anhydropolyamino acid (A-1) in asolvent such as cyclohexane, heptane, methanol, or acetone in which theanhydropolyamino acid (A-1) is insoluble, and then adding thepolymerizable compound (A-2) to the dispersion and mixing; and (4)hydrolyzing the anhydropolyamino acid (A-1) by adding an alkali aqueoussolution to the anhydropolyamino acid (A-1), and then adding thepolymerizable compound (A-2) to the thus obtained aqueous solution andmixing.

The amount of the polymerizable compound (A-2) used should preferably bewithin a range from 0.8 to 3.0 mol, and more preferably be in a rangefrom 0.9 to 2.0 mol per 1 mol of the anhydropolyamino acid (A-1). Whenthe amount of the polymerizable compound (A-2) relative to theanhydropolyamino acid (A-1) is within a range from 0.8 to 3.0 mol, theamount of an unreacted substance can be reduced and formation of aninsoluble substance and coloration due to secondary reaction can beprevented, thereby making it possible to inhibit factors which exert anadverse influence on the product.

When the amount of the polymerizable compound (A-2) is within the aboverange, the amount of an unreacted substance can be reduced and formationof an insoluble substance and coloration due to the crosslinkingreaction as the secondary reaction can be prevented, thereby making itpossible to inhibit factors which exert an adverse influence on theproduct.

Conditions of the reaction between the anhydropolyamino acid (A-1) andthe polymerizable compound (A-2) are not specifically limited, but thereaction is preferably conducted at a temperature within a range from 20to 150° C. The reaction time is preferably two hours or less.

The molecular weight of the resulting polymerizable anhydropolyaminoacid (A) to be used in the present invention is preferably 500 or morein terms of weight-average molecular weight (hereinafter referred to asMw), and more preferably 1000 or more. When the molecular weight is 500or more, a water absorbent material having sufficiently enhanced waterabsorption properties to salts-containing water as the object of thepresent invention can be obtained.

A portion or all of the anhydropolyamino acid (A) is preferablyhydrolyzed. An acidic amino acid residue, which is considered to enhancewater absorption properties for salts-containing water, is formed byhydrolysis. The hydrolysis method is as described below.

As the water-soluble polymerizable monomer (B) used in the presentinvention, for example, there can be used ionic monomer such as(meth)acrylic acid and/or its alkali metal salt, alkali earth metalsalt, or ammonium salt; nonionic monomer such as (meth)acrylamide,N,N-dimethylacrylamide, 2-hydroxyethyl (meth)acrylate, or N-methylol(meth)acrylamide; amino group-containing unsaturated monomer or itsquaternized compound, such as diethylaminethyl (meth)acrylate ordimethylaminopropyl (meth)acrylate; carboxylic acids such as maleicacid, fumaric acid and itaconic acid; monoesters of unsaturateddicarboxylic acid and alcohol, such as monomethyl maleate, monoethylmaleate, monomethyl fumarate, monoethyl fumarate, monomethyl itaconate,and monoethyl itaconate; hydroxyl group-containing (meth)propyl(meth)acrylate such as 2-hydroxyethyl (meth)acrylate or 2-hydroxypropyl(meth)acrylate; hydroxyl group-containing vinyl ethers such as2-hydroxyethyl vinyl ether, 3-hydroxypropyl vinyl ether, and2-hydroxypropyl vinyl ether; sulfonic acid group-containing compound orits alkali metal salt, such as 2-sulfoethyl (meth)acrylate,2-acrylamide-2-methylpropanesulfonic acid,2-methacrylamide-2-methylpropanesulfonic acid, 2-sulfopropyl(meth)acrylate, or 2-sulfopropybutyl (meth)acrylate; and water-solublepolymerizable monomer having an ethylenically unsaturated double bondsuch as alkali earth metal salt or ammonium salt and a sulfonic acidgroup and/or a sulfonate group. These water-soluble polymerizablemonomers can be used alone or in combination.

Examples of the alkali metal salt of (meth)acrylic acid and sulfonicacid include sodium salt, potassium salt, lithium salt, and rubidiumsalt. Among these alkali salts, sodium salt or potassium salt ispreferred in view of the performances of the resulting polymer,industrial availability, and safety.

As used herein, the term “(meth)acrylic” means “acrylic” and“methacrylic”. In these water-soluble polymerizable monomers,(meth)acrylic acid and/or its alkali metal salt, ammonium salt and(meth)acrylamide are preferred in view of water absorption properties.

Among these water-soluble polymerizable monomers, a water-solublepolymerizable monomer having an ethylenically unsaturated double bondsuch as alkali earth metal salt or ammonium salt and a sulfonic acidgroup and/or a sulfonate group (hereinafter referred to as a sulfonicacid group-containing polymerizable monomer) is preferably used becausethe water absorption properties as well as the water absorption ratioand the initial water absorption rate of pure water and water having alow ion content are not lowered.

In addition to the water-soluble polymerizable monomer (B), otherhydrophobic monomers having an ethylenically unsaturated double bond canalso be used as far as water absorption performances of the copolymerconstituting the water absorbent material of the present invention arenot impaired.

Examples of the other hydrophobic monomer having an ethylenicallyunsaturated double bond include various acrylic acid esters such asmethyl acrylate, ethyl acrylate, butyl acrylate, and cyclohexylacrylate; various methacrylic acid esters such as methyl methacrylate,ethyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate,tert-butyl methacrylate, cyclohexyl methacrylate, and benzylmethacrylate; and various diesters of unsaturated carboxylic acid andalcohol, such as dimethyl fumarate, diethyl fumarate, dibutyl fumarate,dioctyl fumarate, dimethyl maleate, diethyl maleate, dibutyl maleate,dioctyl maleate, dimethyl itaconate, diethyl itaconate, dibutylitaconate, and dioctyl itaconate. These hydrophobic monomers can be usedalone or in combination.

The amount of the water-soluble polymerizable monomer (B) is usuallywithin a range from 0.1/1 to 100/1 in terms of a weight ratio[water-soluble polymerizable monomer (B)/polymerizable anhydropolyaminoacid (A)], and preferably from 1/1 to 50/1. When the amount of thewater-soluble polymerizable monomer (B) is within the above range, awater absorbent material having excellent water absorption propertiesfor salts-containing water can be obtained.

The copolymer of the water absorbent material of the present inventionpreferably comprises gel particles having a crosslinked structureintroduced therein to enhance the strength of the copolymer. When thequantity of the crosslinked structure in the copolymer increases, itbecomes possible to enhance the strength of the copolymer. On the otherhand, when the quantity of the crosslinked structure decreases, itbecomes possible to enhance water absorption properties. Therefore, thegel strength and water absorption properties of the copolymer can beadjusted by appropriately controlling the crosslinked structure.

The gel strength of the water absorbent material of the presentinvention is preferably 0.1 g/cm² or more, the upper limitation being anumerical value where desired water absorption properties can beobtained. The gel strength is based on the numerical value measured bythe “method of measuring the gel strength” described hereinafter.

Examples of the method of preparing gel particles include (1) a methodof irradiating the copolymer used in the water absorbent material of thepresent invention with active radiation such as electron beams,radiation or the like and (2) a method of using a crosslinking agent.

Examples of the crosslinking agent include crosslinkable monomers havingat least two ethylenically unsaturated double bonds, crosslinkablemonomer having at least two reactive groups, and crosslinking agentsother than these crosslinking agents.

As the crosslinkable monomer having at least two ethylenicallyunsaturated double bonds, any monomer having two or more ethylenicallyunsaturated double bonds and examples thereof include N,N′-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate,(poly)propylene glycol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerintri(meth)acrylate, glycerin acrylate methacrylate, ethyleneoxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallylcyanurate, triallyl isicyanurate, triallyl phosphate, triallylamine,poly(meth)allyloxyalcane, (poly)ethylene glycol diglycidyl ether,glycerol diglycidyl ether, ethylene glycol, polyethylene glycol,propylene glycol, glycerin, pentaerythritol, ethylenediamine,polyethyleneimine, and glycidyl (meth)acrylate.

Examples of the crosslinkable monomer having at least two reactivegroups include polyhydric alcohol such as ethylene glycol, diethyleneglycol, triethylene glycol, polyethylene glycol, glycerin, polyglycerin,propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,neopentyl alcohol, diethanolamine, tridiethanolamine, polypropyleneglycol, polyvinyl alcohol, pentaerythritol, sorbitol, sorbitan, glucose,manntiol, mannitane, sucrose, or glucose; polyglycidyl ether such asethylene glycol glycidyl ether, polyethylene glycol glycidyl ether, orglycerin triglycidyl ether; haloepoxy compound such as epichlorohydrinor α-methylchlorohydrin; polyaldehyde such as glutalaldehyde orglyoxazol; polyamines such as ethylenediamine; and hydroxide, halide,carbonate, oxide, borate (e.g. borax, etc.) or polyvalent metal compound(e.g. aluminum isopropylate, etc.) of a metal of the group IIA, IIIB andVIII of the Periodic Table, such as calcium hydroxide, calcium chloride,calcium carbonate, calcium oxide, borax magnesium chloride, magnesiumoxide, aluminum chloride, zinc chloride, or nickel chloride.

These compounds can be used alone or in combination, taking reactivityinto consideration.

The amount of the crosslinkable monomer having at least twoethylenically unsaturated double bonds or the crosslinkable monomerhaving at least two reactive groups is preferably within a range from0.005 to 2 mol %, and more preferably from 0.01 to 1 mol %, based on thewater-soluble polymerizable monomer (B). When the amount is within arange from 0.005 to 2 mol %, a water absorbent material having a goodbalance between the water absorption properties and gel strength can beobtained.

Examples of the crosslinking agent include diglycidyl ether compound,haloepoxy compound, polyamine compound, and isocyanate compound.

Examples of the diglycidyl ether compound include ethylene glycoldiglycidyl ether, propylene glycol diglycidyl ether, andglycerin-1,3-diglycidyl ether. Examples of the haloepoxy compoundinclude epichlorohydrin and β-methylepichlorohydrin. Examples of thepolyamine compound include chain aliphatic polyamine such asethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, pentaethylenehexamine, hexamethylenediamine, orpolyether polyamine; cyclic aliphatic polyamine such as menthenediamine,isophoronediamine, orbis(4-aminocyclohxyl)methane-3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxanepyro[5,5]-undecane;aromatic polyamine such as m-xylenediamine or p-xylenediamine;polyamides obtained from dimer acid and an aliphatic polyamine; andbasic amino acid such as lysine.

Examples of the isocyanate compound include tolylene diisocyanate (TDI),phenylene diisocyanate (PPDI), diphenylmethane diisocyanate (MDI),hydrogenated MDI, polymeric MDI, tolidine diisocyanate (TODI),hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI),xylylene diisocyanate (XDI), lysine diisocyanate (LDI),tetramethylenexylene diisocyanate (TMXDI), triphenylmethanetriisocyanate, tris(isocyanatephenyl) thiophosphate, undecanetriisocyanate, lysine ester triisocyanate, 1,8-diisocyanate-4-isocyanatemethyloctane, bicycloheptane triisocyanate, and urethane-modifiedcompounds thereof, alophanate-modified compound, buret-modifiedcompound, isocyanurate-modified compound, carbodimide-modified compound,block isocyanate, and mixtures thereof. These compound may be used aloneor in combination.

The crosslinking agent is preferably used as long as water absorptionproperties of the water absorbent material are not impaired, and theamount is usually within a range from 0.1 to 60 mol %, and preferablyfrom 1 to 50 mol %, based on the imide ring of the anhydropolyaminoacid.

The crosslinkable monomer is used in the reaction between thepolymerizable anhydropolyamino acid (A) and the water-solublepolymerizable monomer (B). The crosslinking agent is preferably addedduring or after the reaction between the polymerizable anhydropolyaminoacid (A) and the water-soluble polymerizable monomer (B).

The water absorbent material comprising a copolymer of the polymerizableanhydropolyamino acid (A), the water-soluble polymerizable monomer (B)and polysaccharides (C) will now be described.

In the copolymer constituting the water absorbent material of thepresent invention, a moiety having high non-ionicity is introduced byusing polysaccharides (C) as a copolymer component, thereby making itpossible to further enhance water absorption properties tosalts-containing water.

Examples of the method of copolymerizing polysaccharides (C) with thepolymerizable anhydropolyamino acid (A) and the water-solublepolymerizable monomer (B) include (1) a method of ring-openingpolysaccharides in the presence of an azo catalyst, thereby to activatecarbon atoms to which hydroxyl groups are attached and tograft-copolymerize carbon atoms with unsaturated double bonds and (2) amethod of using a crosslinking agent capable of reacting with therespective functional groups of the polymerizable anhydropolyamino acid(A), the water-soluble polymerizable monomer (B) and polysaccharides.The copolymer of the water absorbent material of the present inventionobtained by any method can be used.

Examples of polysaccharides (C) include starch, cellulose, and alginicacid.

Examples of the starch include starches made of amylose and/oramylopectin originating in natural substances or plants,starch-containing substances, and modified substances thereof. Specificexamples thereof include potato starch, corn starch, wheat starch,tapioca starch, rice starch, sweet potato starch, sago starch, waxycorns, high amylose corns, wheat flour, and rice flour. As the modifiedstarch, for example, there can be used those obtained by graftcopolymerization of starch and acrylic acid ester, methacrylic acidester, olefin or styrene, those obtained by reacting starch with fattyacid, and those obtained by conversion starch into dextrin or oxidation,pregeklatinization treatment, etherification, esterification orcrosslinking of starch. In addition, a structure-modified starchobtained by heating hydrous starch to a temperature higher than itsglass transition temperature and melting point (described inEP-A-327505) is also included. Furthermore, polysaccharides such as guargum, chitin, chitosan, cellulose, alginic acid, and agar can be used.

Examples of the cellulose include celluloses obtained from woodmaterials, leaves, stems, basts and seed fibers; and processedcelluloses such as alkyl-etherified cellulose, organic acid-esterifiedcellulose, carboxymethylated cellulose, cellulose oxide, andhydroxyalkyl-etherified cellulose.

The amount of polysaccharides (C) is usually 10/1 or less in terms of aweight ratio [polysaccharides (C)/polymerizable anhydropolyamino acid(A)], and preferably 5/1 or less. By using polysaccharides (C) withinthe above range, it becomes possible to obtain the effect that the waterabsorbent material has non-ionicity.

Similar to the water absorbent material, the monomer having at least twoethylenically unsaturated double bonds or the crosslinkable monomerhaving at least two reactive groups and the curing agent can be used asthe monomer component of the copolymer.

The method of preparing the copolymer constituting the water absorbentmaterial of the present invention will now be described.

The method of preparing the copolymer can be conducted by a well-knownmethod. That is, there can be used any method such as (1) a method ofcharging a polymerizable anhydropolyamino acid (A) and a water-solublepolymerizable monomer (B) in a reaction vessel at a time, mixing themand reacting the mixture or (2) a method of initiating the reaction ofone component and adding the other component dropwise. In the presentinvention, this is not specifically limited.

It is preferred in view of uniform reaction that polysaccharides (C) bepreviously dissolved or swollen and dispersed in a system before thereaction.

In the case of reacting the polymerizable anhydropolyamino acid (A) withthe water-soluble polymerizable monomer (B), although a method ofpolymerizing by irradiation with radiation, electron beams, orultraviolet rays, can be employed, a polymerization method using aradical polymerization initiator is industrially preferred. Specificexamples of the radical polymerization initiator include inorganicperoxide such as hydrogen peroxide, ammonium persulfate, potassiumpersulfate, or sodium persufate; organic peroxide such as benzoylperoxide, di-t-butylperoxide, cumenehydroxy peroxide, succinic acidperoxide, or di(2-ethoxyethyl) peroxycarbonate; azo compound such asazobisisobutyronitrile, azobiscyanovaleric acid, or2,2′-azobis(2-aminodipropane) hydrochloride; and redox catalyst (made ofa combination of a reducing agent such as sulfite or hyposulfite ofalkali metal, ammonium sulfite, ammonium bisulfite or ascorbic acid andan oxidizing agent such as persulfate of alkali metal, ammoniumpersulfate, or peroxide. These radical polymerization initiators may beused alone or in combination.

The amount of the radical polymerization initiator is usually within arange from 0.0001 to 5% by weight, and preferably from 0.0005 to 1% byweight, based on the total amount of the water-soluble polymeriablemonomer (B) and the crosslinking agent.

In the polymerization reaction, hydrophilic polymers such as polyacrylicacid or its salt, crosslinked substance thereof, polyvinyl pyrrolidone,and polyvinyl alcohol; chain transfer agents such as hypophosphorousacid and alkylmercaptan; surfactants; and blowing agents such ascarbonate, dry ice, and azo compound can be added.

The polymerization reaction may be conducted in an aqueous solution, asolvent and a suspension, and is not specifically limited.

In the case in which the polymerization reaction is conducted in anaqueous solution, it is preferred to previously hydrolyze thepolymerizable anhydropolyamino acid (A).

The hydrolysis reaction is conducted by an aqueous solution of an alkalimetal compound and/or an alkali earth metal compound under theconditions that the reaction temperature is usually within a range from0 to 100° C., and preferably from 20 to 50° C. The reaction time is notspecifically limited, but is usually 20 hours or less, preferably 10hours or less, and particularly preferably 2 hours or less, in view ofthe productivity.

Typical examples of the alkali metal compound or the alkali earth metalcompound include hydroxide and carbonate of the alkali metal and alkaliearth metal. Specific examples thereof include lithium hydroxide, sodiumhydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide,lithium carbonate, sodium carbonate, potassium carbonate, magnesiumcarbonate, and calcium carbonate. Generally, an aqueous 0.1-40% wt %solution of sodium hydroxide or potassium hydroxide is used. The amountof the alkali compound added is an amount corresponding to 0.4 to 1 molper mol of the imide ring group.

In the case in which the polymerization reaction is conducted in asolvent, the components are dissolved in the solvent. Examples of thesolvent include aprotic organic solvents such as dimethylformamide,dimethylacetamide, N-methylpyrrolidone, N,N-dimethylimidazolione,dimethyl sulfoxide, and sulfolane.

In the case in which the polymerization reaction is conducted in asuspension, a reverse phase suspension polymerization method can beused.

The reverse phase suspension polymerization method will now bedescribed.

Examples of the reverse phase suspension polymerization method include(1) a method of reverse phase suspension polymerization of an aqueousmixed solution of a polymerizable anhydropolyamino acid (A) and awater-soluble polymerizable monomer (B) in a hydrophobic solventcontaining a water-in-oil type (hereinafter referred to as W/O type)surfactant in the presence of a crosslinking agent, using awater-soluble radical polymerization initiator; (2) a method ofinitiating reverse phase suspension polymerization of a water-solublepolymerizable monomer (B) in a hydrophobic solvent containing a W/O typesurfactant in the presence of a crosslinking agent, using awater-soluble radical polymerization initiator, and further conductingreverse phase suspension polymerization by adding dropwise an aqueoussolution of a polymerizable anhydropolyamino acid (A); and (3) a methodof conducting first-stage reverse phase suspension polymerization of anaqueous solution of a water-soluble polymerizable monomer (B) in ahydrophobic solvent containing a W/O type surfactant in the presence ofa crosslinking agent, using a water-soluble radical polymerizationinitiator, and further conducting reverse phase suspensionpolymerization by adding a mixed solution of a polymerizableanhydropolyamino acid (A) and the water-soluble polymerizable monomer(B). In the case of the copolymer constituting the water absorbentmaterial of the present invention, any reverse phase suspensionpolymerization method can be used.

This reverse phase suspension polymerization method is a preferredmethod because a bead-like water absorbent material capable of beingeasily ground can be obtained by using a surfactant.

Specific method of preparing a copolymer by the reverse phase suspensionpolymerization method using a sulfonic acid group-containingpolymeriable monomer as the water-soluble polymerizable monomer (B) willnow be described.

An aqueous solution of a sulfonic acid group-containing polymerizablemonomer (B) is prepared by adding and dissolving a crosslinking agentand a radical polymerization initiator and, if necessary, awater-soluble chain transfer agent such as thiols, thiol acids,secondary alcohols, amines or hypophosphites in an aqueous solutioncontaining a previously neutralized sulfonic acid group-containingpolymerizable monomer and the other monomer having ethylenicallyunsaturated double bonds, and then the resulting solution is deaeratedby introducing an inert gas such as nitrogen. In a polymerizationapparatus, a surfactant is charged in a hydrophobic solvent and isoptionally dissolved by heating, and then aeration is conducted byintroducing a nitrogen gas into the apparatus. The aqueous solution ofthe sulfonic acid group-containing polymerizable polymer is poured intothe apparatus and temperature rising is initiated under stirring. Duringtemperature rising, the aqueous solution is converted into waterdroplets, which are suspended while being dispersed in the hydrophobicsolvent. With the temperature rising, heat is generated and thepolymerization is initiated.

The method of adding the polymerizable anhydropolyamino acid (A) is notspecifically limited, but includes, for example, (1) a method ofpreviously mixing an aqueous solution of a previously hydrolyzedpolymerizable anhydropolyamino acid (A) with an aqueous solution of asulfonic acid group-containing polymerizable monomer, (2) a method ofsimultaneously pouring an aqueous solution of a sulfonic acidgroup-containing polymerizable monomer, (3) a method of pouring duringtemperature rise, or (4) a method of pouring after the polymerizationwas initiated by heat generation. Among these methods, the method (4) ispreferred because it can maintain the stability of the system moresatisfactorily.

In the case in which the aqueous solution of the polymerizableanhydropolyamino acid (A) is added after the polymerization wasinitiated by heat generation, it may be added as it is.

Since there sometimes arise a problem that polymer particles agglomerateaccording to the amount of the polymerizable anhydropolyamino acid (A),an inert solvent containing a surfactant dissolved therein is added toan aqueous solution of the polymerizable anhydropolyamino acid (A) andthe polymerizable anhydropolyamino acid (A) is dispersed in the inertsolvent by stirring, and then the resulting dispersion is preferablyadded to a polymer solution.

Although timing of pouring after heat generation is not specificallylimited, pouring is conducted during the time immediately after heatgeneration and the time after three hours have passed since heatgeneration, and particularly preferably during the time immediatelyafter heat generation and the time after two hours have passed sinceheat generation. Pouring is preferably conducted during the above timingrange because the aqueous solution of the polymerizable anhydropolyaminoacid (A) can be incorporated into particles of the copolymer, which isbeing formed in the reaction system, without separating the aqueoussolution.

After initiation of the polymerization, cooling is appropriatelyconducted according to the state of heat generation. The temperature ofthe polymerization reaction is preferably within a range from 60 to 100°C., and particularly preferably from 60 to 80° C.

The rotation number of a stirring blade of a reaction apparatus duringthe reaction cannot be shown unambiguously because the absolute valuethereof varies depending on the kind of the stirring blade and the sizeof the polymerization reaction vessel, but is preferably within a rangefrom 50 to 500 rpm in view of the polymerization safety.

The suspension polymerization reaction yields a slurry mixturecontaining particles having an average particle diameter within a rangefrom 10 to 300 μm (hydrous gel particles/excess surfactant/hydrophobicsolvent).

The slurry mixture is converted into copolymer particles by directdehydration according to a publicly known procedure or azeotropicdehydration with a hydrophobic solvent and optionally subjecting to asurface treatment, followed by drying and various steps such asscreening.

As the W/O type surfactant used in the present invention, any surfactantcan be used as long as it is soluble in a hydrophobic solvent or it hashydrophilicity and is capable of forming a W/O type emulsion system.

The surfactant having such properties is a nonionic or anionicsurfactant which generally has a HLB value within a range from 1 to 9,and preferably from 2 to 7. Specific examples of the surfactant includesorbitan fatty acid ester, polyoxysorbitan fatty acid ester, sucrosefatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkylphenyl ether, ethylcellulose, ethylhydroxyethylcellulose, polyethyleneoxide, maleic anhydride modified polyethylene, maleic anhydride modifiedpolybutadiene, maleic anhydride modifiedethylene-propylene-diene-terpolymer, copolymer of α-olefin and maleicanhydride or derivative thereof, and polyoxyethylene alkyl etherphosphoric acid.

The amount of the surfactant is within a range from 0.05 to 10% byweight, and preferably from 0.1 to 1% by weight.

As the hydrophobic solvent, any solvent can be used as long as it isbasically insoluble in water and is inert with respect to thepolymerization reaction. Examples thereof include aliphatic hydrocarbonssuch as n-pentane, n-hexane, n-heptane, or n-octane; and aromatichydrocarbons such as benzene, toluene, or xylene. Among these solvents,n-hexane, n-heptane and cyclohexane are particularly preferred because anon-greasy water absorbent material can be obtained.

The amount of these hydrophobic solvents is usually 0.5 to 10 timeslarger than that of the aqueous solution of the water-solublepolymerizable monomer (B) used in the first-stage reaction in thereverse phase suspension polymerization method (3).

The reaction operation may be conducted in atmospheric air, but ispreferably conducted in an inert gas atmosphere to inhibit the secondaryreaction. The reaction pressure is not specifically limited, but ispreferably a reduced pressure below normal pressure, and particularlypreferably is a highly reduced pressure. Specifically, it is preferablywithin a range from 10 to 1.013×10⁵ Pa.

The reaction time is not specifically limited, but is usually 100 hoursor less, preferably 50 hours or less, and more preferably 20 hours orless.

The method of preparing the copolymer used in the water absorbentmaterial of the present invention using the reaction apparatus includes,for example, a preparation method using a publicly known reactionapparatus and specific examples thereof include (1) a method ofpolymerizing while stirring optionally in a twin-bowl kneader, (2) amethod of cast-polymerizing in a container and (3) a method ofstanding-polymerizing by continuously feeding on a driving belt. Thewater absorbent material of the present invention can be prepared by anymethod, in addition to the preparation methods using the apparatus.

To optimize water absorption properties, about 5 to 100 mol %,preferably 65 to 80 mol % of acid groups in the water-solublepolymerizable monomer (B) is preferably neutralized with an alkalicompound before or after the reaction.

The alkali compound used herein is preferably a hydroxide or a carbonateof an alkali metal salt or an alkali earth metal salt. Examples of thecompound include lithium hydroxide, sodium hydroxide, potassiumhydroxide, magnesium hydroxide, calcium hydroxide, lithium carbonate,sodium carbonate, potassium carbonate, magnesium carbonate, and calciumcarbonate.

The water absorbent material of the present invention varies dependingon the method of the copolymerization reaction of the copolymer. In caseof the reaction of an aqueous solution, the water absorbent material canbe prepared by granulating hydrous gel particles made of the copolymerafter the reaction, and passing through a series of steps such as dryingstep, grinding step, screening step, surface crosslinking treatment stepand screening step.

To dry hydrous gel particles obtained through the above steps, hydrousgel particles are preferably granulated to form granules having apredetermined particle diameter in order to enhance the dryingefficiency by increasing the surface area. Hydrous gel particles aregranulated simultaneously by polymerizing while stirring with atwin-bowl kneader, or by extruding polymerized hydrous gel particlesthrough a die using a meat grinder. Hydrous gel particles can also begranulated by a cutting mill. Although the particle diameter ofgranulated gel particles can be appropriately adjusted by a drier, butan average particle diameter is preferably within a range from 0.1 to 10mm. When the average particle diameter is less than 0.1 mm, physicalproperties of the water absorbent material are likely to be lowered. Onthe other hand, when the average particle diameter exceeds 10 mm,granulated gel particles are not easily dried and, therefore, it is notpreferred.

On granulation of hydrous gel particles made of the copolymer, gelcoarse particles having an average particle diameter of larger than 10mm and gel fine particles having an average particle diameter of smallerthan 0.1 mm are likely to be formed. The gel coarse particles and gelfine particles can be reused by adding to the aqueous solution of thewater-soluble polymerizable monomer (B) and polymerized gel particlesafter screening and recovering.

Gel particles granulated in the granulating step are dried in thefollowing drying step. In the drying method, for example, a hot-airdrier, air-current drier, a fluidized bed drier, a drum drier, microwavedrier, a far infrared drier and a vacuum drier can be appropriatelyused.

Hydrous gel particles obtained in the drying step are ground andscreened according to uses of the water absorbent material in thefollowing grinding step and screening step to form granules having apredetermined particle size. When used in a diaper, napkin or the like,the screened granules have a particle size of 1 mm or less, andpreferably 0.85 mm or less. To sufficiently exert water absorptionperformances in the diaper or sanitary napkin, fine powders having aparticle diameter of 105 μm or less, preferably 212 μm or less, and morepreferably 300 μm or less are preferably removed by screening. Finepowders recovered in the grinding step and screening step can be reusedby adding in the polymerizing step and drying step.

The vicinity of the surface of the water absorbent material of thepresent invention can be crosslinked by using a surface crosslinkingagent having two or more functional groups capable of reacting withfunctional groups of the copolymer constituting the water absorbentmaterial.

Specifically, the water absorbent material of the present invention ismixed with the surface crosslinking agent, and then the vicinity of thesurface of the water absorbent material is crosslinked by a heattreatment.

Examples of the surface crosslinking agent include polyhydric alcoholsuch as diethylene glycol, propylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropyleneglycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,polyglycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanediol,trimethlolpropane, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, pentaerythritol, or sorbitol;epoxy compound such as ethylene glycol diglycidyl ether, polyethyleneglycol diglycidyl ether, glycerol polyglycidyl ether, diglycerolpolyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycoldiglycidyl ether, polypropylene glycol diglycidyl ether, or glycidol;polyhydric amine compound such as ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine,polyethyleneimine, or polyamidepolyamine; haloepoxy compound such asepichlorohydrin, epibromohydrin, or α-methylepichlorohydrin; condensateof the above polyvalent amine compound and the above haloepoxy compound;polyhydric isocyanate compound such as 2,4-tolylene diisocyanate orhexamethylene diisocyanate; polyhydric oxazoline compound such as1,2-ethylenebisoxazoline; silane coupling agent such asγ-glycidoxypropyltrimethoxysilane or γ-aminopropyltrimethoxysilane;alkylene carbonate compound such as 1,3-dioxolan-2-one,4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one,4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one,4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one,4-methyl-1,3-dioxan-2-one, 4,6-dime 1,3-dioxan-2-one, 1,3-dioxan-2-one,or 1,3-dioxoban-2-one; and polyhydric metal compound such as hydroxideor chloride of zinc, calcium, magnesium or aluminum. Among these surfacecrosslinking agents, polyhydric alcohol compound, epoxy compound,polyhydric amine compound, condensate of polyvalent amine compound andhaloepoxy compound, and alkylene carbonate compound are preferred inview of the reactivity and safety. These surface crosslinking agents maybe used alone or in combination.

The amount of the surface crosslinking agent relative to the waterabsorbent material varies depending on the combination of the waterabsorbent material and the surface crosslinking agent, but is usuallywithin a range from 0.01 to 10 parts by weight, and preferably from 0.05to 3 parts by weight, based on 100 parts by weight of the copolymer inthe dry state. By using the surface crosslinking agent in the amountwithin the above range, water absorption properties for body fluids(aqueous liquids) such as urine, sweat or menstrual blood can be furtherimproved.

In the case in which the water the water absorbent material is mixedwith the surface crosslinking agent, water is preferably used. Theamount of water varies depending on the kind, the particle size and thewater content of the water absorbent material, but is usually within arange from 0.5 to 10 parts by weight, and preferably from 0.5 to 3 partsby weight, based on 100 parts by weight of the solid content of thewater absorbent material, thus making it possible to form a crosslinkinglayer having a sufficient thickness in the vicinity of the surface.

In the case in which the water the water absorbent material is mixedwith the surface crosslinking agent, a hydrophilic organic solvent maybe used. Examples of the hydrophilic organic solvent include loweralcohol such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropylalcohol, butyl alcohol, isobutyl alcohol, or t-butyl alcohol; ketonessuch a acetone; ethers such as dioxane, alkoxy (poly)ethylene glycol, ortetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxidessuch as dimethyl sulfoxide.

The amount of the hydrophilic organic solvent varies depending on thekind and particle size of the water absorbent material, but is usuallywithin a range from 0.001 to 10 parts by weight, and preferably from 0.1to 5 parts by weight, based on 100 parts by weight of the waterabsorbent material.

It is necessary that a preferred mixer used to mix the surfacecrosslinking agent with the water absorbent material is capable ofproducing a large mixing force to ensure uniform mixing. Preferredexamples of the mixer include a cylindrical mixer, double wall conicalmixer, high-speed stirring mixer, V-shaped mixer, ribbon-shaped mixer,screw mixer, flow furnace rotary disk type mixer, air-current mixer,twin-bowl kneader, internal mixer, grinding mixer, rotary mixer, andscrew extruder.

The temperature of the heat treatment is preferably within a range from80 to 300° C. When the temperature is within the above range, moreuniform crosslinking can be achieved, thereby making it possible toobtain a water absorbent material, which is capable of reducing theamount of a soluble component eluted and has excellent water absorptionproperties.

As the apparatus for heat treatment, for example, a publicly known drieror heating furnace can be used. Specific examples thereof include groovemixing drier, rotary drier, disk drier, fluidized bed drier, air-currentdrier, infrared drier, and vacuum drier.

The water absorbent material of the present invention is superior inwater absorption properties to pure water and salts-containing water,especially water absorption properties to salts-containing water.

Water absorption performances can be determined by the method of testingwater absorption of a high water absorbent material using a tea bagmethod defined in Japanese Industrial Standard (JIS K7223). In the caseof evaluating by the tea bag method, the water absorbent material of thepresent invention has water absorption performances of 20 times or moreto ion exchange water and has water absorption performances of 5 timesor more to a physiological saline (aqueous 0.9% sodium chloridesolution).

The water absorbent material of the present invention can be applied toall purposes that have conventionally been known. The water absorbentmaterial can be used in the fields, for example, sanitary field such assanitary products (e.g. diapers, sanitary napkins, etc.), medical fieldsuch as poultices, civil engineering and construction field such assludge gelling agents, food field, industrial field, andagricultural/horticultural field such as soil conditioners andwater-storing materials and its utility value is remarkably great. Otherpurposes include water-swelling rubber prepared by incorporating arubber into the water absorbent material.

The absorbent article of the present invention will now be described.

The liquid-permeable sheet constituting the absorbent article of thepresent invention means a sheet made of a material which is permeable toan aqueous liquid, and examples thereof include nonwoven fabric, wovenfabric, and synthetic films made of a material such as polyethylene,polypropylene, or polyamide.

The liquid-impermeable sheet constituting the absorbent article of thepresent invention means a sheet made of a material which is impermeableto an aqueous liquid, and examples thereof include synthetic films madeof a material such as polyethylene, polypropylene, ethylene vinylacetate, or polyvinyl chloride, and a film made of a composite of thesynthetic resin and a nonwoven fabric or a woven fabric.

The liquid-impermeable sheet constituting the absorbent article of thepresent invention can use the above water absorbent material.

Examples of the fiber material constituting the water absorbent materialinclude hydrophobic fiber material and hydrophilic fiber material. Amongthese fiber materials, hydrophilic fiber material is preferred in viewof superior affinity with the solution to be absorbed. Examples of thehydrophilic fiber material include cellulose fiber such as mechanicalpulp or semi-chemical pulp obtained from wood materials; artificialcellulose fiber such as rayon or acetate; and fiber material obtained byhydrophilization of thermoplastic fiber. The fiber material may have afibrous shape, or may be formed into a sheet such as tissue paper orpulp mat.

The absorbent article of the present invention comprises aliquid-permeable sheet, a liquid-impermeable sheet, and an absorbercomprising the water absorbent material, and a fiber material arrangedbetween the liquid-permeable sheet and the liquid-impermeable sheet, andhas a structure such that the absorber is supported inside. Specificexamples of the method of producing the absorbent article include amethod of sandwiching the absorber between the liquid-permeable sheetand the liquid-impermeable sheet and bonding an outer peripheral portionof the liquid-permeable sheet and the liquid-impermeable sheet using anadhesive such as hot-melt adhesive or a bonding means such as a heatseal.

The method of producing an absorber comprising a water absorbentmaterial and a fiber material is not specifically limited, but includes(1) a method of forming a fiber material into a sheet and covering awater absorbent material with the sheet, (2) a method of dispersing awater absorbent material over a multi-layer fiber sheet and forming themulti-layer sheet, and (3) a method of mixing a fiber material with awater absorbent material and forming the mixture into a sheet.

The absorbent article of the present invention is used in disposablediapers for infants, adults, and persons suffering from incontinence,and sanitary napkins. The absorbent article is particularly suited foruse in disposable diapers for adults, wherein a swelling gel of thewater absorbent material is drastically deteriorated due to a largeamount of excretion and long time in contact with urine, among theseuses because of its excellent absorbency of urine and body fluids aswell as excellent urine leakage inhibition effect.

EXAMPLES

The following Examples further illustrate the present invention indetail, but the present invention is not limited by the Examples. In thefollowing Examples, percentages are by weight unless otherwisespecified. Various characteristics of the resin of the present inventionwere determined by the following methods.

[Method of Measuring Water Absorption Ratio]

The water absorption capability of resins obtained in the Examples ofthe present invention was determined in accordance with the method oftesting water absorption of a high water absorbent material described inJapanese Industrial Standard JIS K7223. 0.20 g (1.00 g based on anaqueous 0.9% sodium chloride solution) of a dry resin was put in a teabag (200 mm×100 mm) made of a 255 mesh nylon gauze and the resin wasswollen by dipping in 1000 ml of ion exchange water or an aqueous 0.9%sodium chloride solution for a fixed time. After pulling out the teabag, the solution was drained off and the weight was measured. The sameoperation was repeated, except that only the tea bag was used and theweight of the tea bag was measured. The resulting weight was taken asthe blank. The water absorption ratio W (g/g) was calculated inaccordance with the following equation:$W = \frac{b_{60\quad\min} - c - a}{a}$where W is water absorption ratio (g/g), a is a weight (g) of a sample,b is a weight (g) of the sample measured after a tea bag containing thesample was dipped for a predetermined time and the solution was drainedoff, and c is an average value of a weight (g) of the sample measuredafter a tea bag containing no sample was dipped for a predetermined timeand the solution was drained off.[Method of Measuring Water Absorption Rate]

A stirrer tip was rotated at about 600 rpm in 50 g of an aqueous 0.9%sodium chloride solution in a 100 ml glass beaker and then 2 g of asample was put in the beaker along with the inner wall thereof. Then, aresin was added and the time (in seconds), which is required for theresin to be swollen by water absorption, thereby stopping rotation ofthe stirrer tip, was taken as the water absorption rate.

[Method of Measuring Gel Strength]

1.0 g of a crosslinked resin was allowed to absorb 100 g of pure water(water absorption by 100 times) and a dead-weight was placed on theresin after water absorption. The total weight of the dead-weight perunit area (g/cm²) when the dead-weight placed first on the resin wastaken as a gel strength.

Synthetic Example 1

In a 2 L round bottom flask, 100 g of L-aspartic acid and 50 g of 85%phosphoric acid were charged and the reaction was conducted underreduced pressure in an oil bath at a bath temperature of 200° C. forfour hours using an evaporator. The resulting product (25 g) was washedseveral times with water and methanol to obtain polysuccinimide. As aresult of the measurement of gel permeation chromatography (hereinafterreferred to as GPC), this polysuccinimide has a Mw of 20,000.

Synthetic Example 2

In a 1 L four-necked flask equipped with a stirrer, a thermometer, areflux condenser, and a nitrogen gas introducing device, 96 g of maleicanhydride and 50 g of ion exchange water were added and, afterdissolving maleic anhydride by heating to 55° C., the solution wascooled to obtain a slurry of maleic anhydride. When the temperature ofthe system reached 55° C. as a result of heating, 60.8 g of 28% ammoniawater was added. The system was heated to 80° C. and, after the reactionwas conducted for three hours, the resulting aqueous solution was driedto obtain a reaction intermediate. In a 2 L round bottom flask, 100 g ofthe reaction intermediate and 10 g of 85% phosphoric acid were chargedand the reaction was conducted under reduced pressure in an oil bath ata bath temperature of 200° C. for four hours using an evaporator. Theresulting product was washed several times with water and methanol toobtain polysuccinimide. The Mw of this polysuccinimide was measured byGPC. As a result, it was 3,000.

Example 1

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser and a nitrogen gas introducing device, 10 g ofpolysuccinimide obtained in Synthetic Example 1 and 20 g ofN,N-dimethylformamide (hereinafter referred to as DMF) were charged anddissolved at about 60° C., and furthermore, 0.5 g of2-methacryloyloxyethyl isocyanate was added. As a result, heat wasgenerated. After the reaction was continued for 30 minutes, 150 g of anaqueous solution containing 3.3 g of sodium hydroxide dissolved thereinwas added and polysuccinimide was hydrolyzed. After the temperature inthe system was reduced to about 35° C., 1 g of “ALSTAR B”(pregelatinized starch, manufactured by NIHON SHOKUHIN KAKO CO., LTD.),25 g of acrylic acid and 0.25 g of dipentaerythritol hexamethacrylatewere charged. After the atmosphere in the system was replaced by anitrogen gas, 7.5 mg of 2,2-azobisaminodipropane dihydrochloride, 5 mgof ascorbic acid and 57 mg of an aqueous hydrogen peroxide (35%)solution were respectively dissolved in 1 g of ion exchange water. Theresulting solutions were added to the above aqueous solution in thissequence and the reaction was initiated, and then the mixture wasmaintained at about 60° C. for three hours (hereinafter referred to as(1) first step). 2.4 g of hexamethylenediamine was dissolved in 20 g ofion exchange water and the resulting solution was added to the aboveaqueous solution, and then the reaction was conducted. Furthermore, 10.4g of sodium hydroxide was dissolved in 30 g of ion exchange water andthe resulting solution was added, thereby to neutralize carboxyl groupsoriginating in acrylic acid (hereinafter referred to as (1) third step).The resulting gel-like product was dried at 110° C. by a vacuum drierand the resulting dry solid was ground to obtain a water absorbentmaterial of the present invention. The evaluation results ofcharacteristics of the resulting water absorbent material are shown inTable 2.

Example 2

In the same operation and manner as in Example 1, except that the amountof acrylic acid was changed to 30 g, the amount of dipentaerythritolhexamethacrylate was changed to 0.1 g, and the amount of sodiumhydroxide for neutralization of carboxyl groups originating in acrylicacid was changed to 12.5 g, a water absorbent material of the presentinvention was obtained. The evaluation results of characteristics of theresulting water absorbent material are shown in Table 2.

Example 3

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser, a dropping funnel and a nitrogen gas introducingdevice, 10 g of polysuccinimide obtained in Synthetic Example 2 and 10 gof DMF were charged and dissolved at about 60° C., and then 0.5 g of2-methacryloyloxyethyl isocyanate was added. As a result, heat wasgenerated. After the reaction was continued for 30 minutes, 125 g of anaqueous solution containing 2.1 g of sodium hydroxide dissolved thereinwas added and polysuccinimide was hydrolyzed. After the temperature inthe system was reduced to about 35° C., 1 g of “ALSTAR B” was charged.

In a 100 ml Erlenmeyer flask, 25 g of acrylic acid, 0.125 g ofdipentaerythritol hexamethacrylate and 0.04 g of “THIOKALCOL 20”(laurylmercaptane, manufactured by Kao Corp.) were mixed to prepare anacrylic acid solution. 5 g of this mixed solution was added to thefour-necked flask and the rest was transferred to a dropping funnel.After the atmosphere in the system was replaced by a nitrogen gas, 7.5mg of 2,2-azobisaminodipropane dihydrochloride, 5 mg of ascorbic acidand 57 mg of an aqueous hydrogen peroxide (35%) solution wererespectively dissolved in 1 g of ion exchange water. The resultingsolutions were added to the above aqueous solution ((1) first step).After 10 minutes, dropwise addition of the acrylic acid solution in thedropping funnel was initiated and was completed over one hour. After onehour has passed since the completion of the reaction, 7.5 g of2,2-azobisaminodipropane dihydrochloride, 5 mg of ascorbic acid and 57mg of an aqueous hydrogen peroxide (35%) solution were respectivelydissolved again in 1 g of ion exchange water. The resulting solutionswere added to the above aqueous solution in this sequence, and then themixture was heated to about 60° C. and maintained at the sametemperature for three hours (hereinafter referred to as (1) secondstep).

2.4 g of hexamethylenediamine was dissolved in 20 g of ion exchangewater and the resulting solution was added to the above aqueoussolution, and then the reaction was conducted. Furthermore, 10.4 g ofsodium hydroxide was dissolved in 30 g of ion exchange water and theresulting solution was added, thereby to neutralize carboxyl groupsoriginating in acrylic acid ((1) third step). The resulting gel-likeproduct was dried at 110° C. by a vacuum drier and the resulting drysolid was ground to obtain a water absorbent material of the presentinvention. The evaluation results of characteristics of the resultingwater absorbent material are shown in Table 2.

Example 4

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser, a dropping funnel and a nitrogen gas introducingdevice, 5 g of polysuccinimide obtained in Synthetic Example 2 and 10 gof DMF were charged and dissolved at about 60° C., and then 0.25 g of2-methacryloyloxyethyl isocyanate was added. As a result, heat wasgenerated. After the reaction was continued for 30 minutes, thetemperature in the system was reduced to about 35° C., 1 g of apregelatinized starch was charged. After the atmosphere in thefour-necked flask was replaced by a nitrogen gas, 7.5 mg of2,2-azobisaminodipropane dihydrochloride, 5 mg of ascorbic acid and 57mg of an aqueous hydrogen peroxide (35%) solution were respectivelydissolved in 1 g of ion exchange water. The resulting solutions wereadded to the above aqueous solution, and then the mixture was heated toabout 60° C. and maintained for three hours ((1) first step). To thepolymer obtained by this step, 25 g of acrylic acid and 25 mg ofN,N′-methylenebisacrylamide were added. After the atmosphere in thefour-necked flask was replaced by a nitrogen gas, 7.5 g of2,2-azobisaminodipropane dihydrochloride, 5 mg of ascorbic acid and 57mg of an aqueous hydrogen peroxide (35%) solution were respectivelydissolved again in 1 g of ion exchange water. The resulting solutionswere added to the above aqueous solution in this sequence, and then themixture was heated again to about 60° C. and maintained at the sametemperature for three hours ((1) second step). 1.2 g ofhexamethylenediamine was dissolved in 20 g of ion exchange water and theresulting solution was added to the above aqueous solution, and then thereaction was conducted. Furthermore, 10.4 g of sodium hydroxide wasdissolved in 30 g of ion exchange water and the resulting solution wasadded, thereby to neutralize carboxyl groups originating in acrylic acid((1) third step). The resulting gel-like product was dried at 110° C. bya vacuum drier and the resulting dry solid was ground to obtain a waterabsorbent material of the present invention. The evaluation results ofcharacteristics of the resulting water absorbent material are shown inTable 2.

Comparative Example 1

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser and a nitrogen gas introducing device, 1 g of apregelatinized starch, 25 g of acrylic acid, 0.25 g of dipentaerythritolhexamethacrylate and 150 g of ion exchange water were charged. After theatmosphere in the system was replaced by a nitrogen gas, 7.5 mg of2,2-azobisaminodipropane dihydrochloride, 5 mg of ascorbic acid and 57mg of an aqueous hydrogen peroxide (35%) solution were respectivelydissolved in 1 g of ion exchange water. The resulting solutions wereadded in this sequence and the reaction was initiated, and then themixture was maintained at 60° C. for three hours ((1) first step). Afterthe completion of the reaction, 10.4 g of sodium hydroxide was dissolvedin 30 g of ion exchange water and the resulting solution was added,thereby to neutralize carboxyl groups originating in acrylic acid ((1)third step). The resulting gel-like product was dried at 110° C. by avacuum drier and the resulting dry solid was ground. The evaluationresults of characteristics of the resulting ground material are shown inTable 2.

Components to be charged of Examples 1 to 4 and Comparative Example 1are shown in Table 1.

TABLE 1 Comp. Components to be charged (g) Example 1 Example 2 Example 3Example 4 Example 1 Polysuccinimide (1) 10 10 — — — Polysuccinimide (2)— — 10 5 — (1) First step DMF 20 20 10 10 — MCOEI 0.5 0.5 0.5 0.25 —NaOH 3.3 3.3 2.1 — — Ion exchange water 146.7 146.7 122.9 — 150.0Pregelatinized starch 1 1 1 1 1 Acrylic acid 25 30 25 — 25 DPMA 0.25 0.10.125 — 0.25 Laurylmercaptane — — 0.04 — — ABAPHC 0.0075 0.0075 0.00750.0075 0.0075 Ascorbic acid 0.005 0.005 0.005 0.005 0.005 35% hydrogenperoxide water 0.057 0.057 0.057 0.057 0.057 Ion exchange water 3 3 3 33 (1) Second step Acrylic acid — — — 25 — MBAA — — — 0.025 — ABAPHC — —0.0075 0.0075 — Ascorbic acid — — 0.005 0.005 — 35% hydrogen peroxidewater — — 0.057 0.057 — Ion exchange water — — 3 3 — (1) Third stepHexamethylenediamine aqueous solution 2.4 2.4 2.4 1.2 — (ion exchangewater: 20 g) NaOH aqueous solution 10.4 12.5 10.4 10.4 10.4 (ionexchange water: 30 g)

-   Polysuccinimide (1): polysuccinimide (Mw: 20,000) obtained in    Synthetic Example 1-   Polysuccinimide (2): polysuccinimide (Mw: 3,000) obtained in    Synthetic Example 2-   MCOEI: 2-methacryloyloxyethyl isocyanate-   DPMA: dipentaerythritol hexamethacrylate-   ABAPHC: 2,2-azobisaminodipropane dihydrochloride-   MBAA: N,N′-methylenebisacrylamide

TABLE 2 Water absorption ratio (g/g) 0.9% aqueous solution of sodium Ionexchange water chloride Example 1 230 62 Example 2 270 70 Example 3 237114 Example 4 301 107 Comparative Example 1 400 39

Example 5

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser, a dropping funnel and a nitrogen gas introducingdevice, 121 g of cyclohexane was added, and then 0.9 g of sorbitanmonostearate was added thereto and dissolved by heating to 50° C. whilestirring. The contents in the flask were cooled to 30° C.

In a 500 ml Erlenmeyer flask, 30 g of acrylic acid was charged andneutralized (75 mol %) by adding dropwise 91.8 g of an aqueous sodiumhydroxide solution containing 12.5 g of sodium hydroxide dissolvedtherein while cooling from outside. To this solution, 21 mg ofN,N′-methylenebisacrylamide was added and then 0.104 g of potassiumpersulfate and 0.0426 g of sodium hypophosphite monohydrate were addedand dissolved. The resulting partially neutralized aqueous acrylic acidsalt solution containing a polymerization initiator and a crosslinkingagent (neutralization degree: 75 mol %) was added in the contents of theabove cylindrical round bottom flask and dispersed in the cyclohexanesolution containing the surfactant, and then the atmosphere in thesystem was sufficiently replaced by nitrogen. A bath was heated and thetemperature was set to 70° C. and the mixture was maintained at the sametemperature for three hours, and then the polymerization reaction wasconducted (hereinafter referred to as (2) first step). Thepolymer-containing slurry solution obtained by this step was cooled to20° C.

In a separate 500 ml four-necked flask equipped with a stirrer, athermometer, a reflux condenser, and a nitrogen gas introducing device,7 g of polysuccinimide obtained in Synthetic Example 2 and 10 g of DMFwere charged and dissolved at about 60° C., and then 0.35 g of2-methacryloyloxyethyl isocyanate was added. As a result, heat wasgenerated. After the reaction was continued for 30 minutes, 10 g of anaqueous sodium hydroxide solution containing 1.44 g of sodium hydroxidedissolved therein was added, thereby hydrolyzing polysuccinimide(hereinafter referred to as (2) second step). In the same manner asdescribed above, 20.3 g of an aqueous partially neutralized sodiumacrylate solution (neutralization degree: 75 mol %) was prepared from 5g of the aqueous acrylic acid solution and then added to the abovesolution. Furthermore, 1.73 g of potassium persulfate, 21 mg ofN,N′-methylenebisacrylamide and 7.1 mg of sodium hypophosphitemonohydrate were added and dissolved. The aqueous monomer solution thusobtained was transferred to the dropping funnel and then slowly addeddropwise to the polymer slurry solution maintained at 20° C. over 30minutes. After the atmosphere in the flask was sufficiently replaced bynitrogen, the contents were heated to 70° C. and maintained at the sametemperature for three hours, and furthermore, the polymerizationreaction was conducted (hereinafter referred to as (2) third step).Under reduced pressure, cyclohexane and water were removed. Theevaluation results of characteristics of the resulting water absorbentmaterial as gel-like particles are shown in Table 4.

Comparative Example 2

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser, a dropping funnel, and a nitrogen gas introducingdevice, 121 g of cyclohexane was added, and then 0.9 g of sorbitanmonostearate was added thereto and dissolved by heating to 50° C. whilestirring. The contents in the flask were cooled to 30° C. In a 500 mlErlenmeyer flask, 30 g of acrylic acid was charged and neutralized (75mol %) by adding dropwise 91.8 g of an aqueous sodium hydroxide solutioncontaining 12.5 g of sodium hydroxide dissolved therein while coolingfrom outside. To this solution, 21 mg of N,N′-methylenebisacrylamide wasadded and then 0.104 g of potassium persulfate and 0.0426 g of sodiumhypophosphite monohydrate were added and dissolved. The resultingpartially neutralized aqueous acrylic acid salt solution containing apolymerization initiator and a crosslinking agent (neutralizationdegree: 75 mol %) was added in the contents of the above cylindricalround bottom flask and dispersed in the cyclohexane solution containingthe surfactant, and then the atmosphere in the system was sufficientlyreplaced by nitrogen. A bath was heated and the temperature was set to70° C. and the mixture was maintained at the same temperature for threehours, and then the polymerization reaction was conducted (hereinafterreferred to as (2) first step).

In the same manner as described above, 20.3 g of an aqueous partiallyneutralized sodium acrylate solution (neutralization degree: 75 mol %)was prepared from 5 g of the aqueous acrylic acid solution in a 100 mlErlenmeyer flask and was then added. Furthermore, 1.73 mg of potassiumpersulfate, 21 mg of N,N′-methylenebisacrylamide and 7.1 mg of sodiumhypophosphite monohydrate were added and dissolved. The aqueous monomersolution thus obtained was transferred to the dropping funnel and thenslowly added dropwise to the polymer slurry solution maintained at 20°C. over 30 minutes. After the atmosphere in the flask was sufficientlyreplaced by nitrogen, the contents were heated to 70° C. and maintainedat the same temperature for three hours, and furthermore, thepolymerization reaction was conducted ((2) third step). Under reducedpressure, cyclohexane and water were removed. The evaluation results ofcharacteristics of the resulting water absorbent material as gel-likeparticles are shown in Table 4.

Components to be charged of Example 5 and Comparative Example 2 areshown in Table 3.

TABLE 3 Comp. Components to be charged (g) Example 5 Example 2 (2) Firststep Cyclohexane 121 121 Sorbitan monostearate 0.9 0.9 Acrylic acid 3030 NaOH 12.5 12.5 Ion exchange water 79.3 79.3 MBAA 0.021 0.021Potassium persulfate 0.104 0.104 Sodium hypophosphite monohydrate 0.04260.0426 (2) Second step Polysuccinimide (2) 7 — DMF 10 — MCOEI 0.35 —NaOH 1.44 — Ion exchange water 8.56 — (2) Third step 75 mol % partiallyneutralized sodium 20.3 20.3 acrylate Potassium persulfate 0.001730.00173 MBAA 0.021 0.021 Sodium hypophosphite monohydrate 0.0071 0.0071MBAA: N,N′-methylenebisacrylamide Polysuccinimide (2): polysuccinimide(Mw: 3,000) obtained in Synthetic Example 2 MCOEI:2-methacryloyloxyethyl isocyanate

TABLE 4 Water absorption ratio (g/g) Ion exchange water 0.9% aqueoussolution of sodium chloride Example 5 615 135 Comp. Example 2 676 67

Example 6

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser and a nitrogen gas introducing device, 75 g of anaqueous solution containing 20.6 g of sodium hydroxide dissolved thereinwas added and then 50 g of a powder of polysuccinimide obtained inSynthetic Example 2 was added to prepare an aqueous solution ofpolysuccinimide. After the temperature was raised to 90° C., 5.0 g ofglycidyl methacrylate was added and the reaction was conducted for onehour to obtain an aqueous solution containing a hydrolysate ofpolysuccinimide having methacryloyl groups introduced therein.

In a 100 ml Erlenmeyer flask, 0.75 g of “DK ESTER F-160” [sucrose fattyacid ester (HLB=16), manufactured by DAI-ICHI KOGYO SEIYAKU CO., LTD.]was weighed and dissolved by adding 29 g of cyclohexane and heating to50° C. The resulting solution was added to 7.7 g of the aqueous solutionobtained by the above operation, followed by stirring to obtain adispersion of polysuccinimide having methacryloyl groups introducedtherein in an aqueous hydrolyzed solution (hereinafter referred to as(3) first step).

In a separate 500 ml four-necked flask equipped with a stirrer, athermometer, a reflux condenser, and a nitrogen gas introducing device,164 g of cyclohexane was added, and then 0.75 g of “DK ESTER F-90” wasadded and dissolved by heating to 50° C. with stirring. The contents inthe flask were cooled to 30° C. In a 500 ml Erlenmeyer flask, 18.4 g ofsodium sulfoethylmethacrylate was added. To this solution, 18.4 g ofacrylamide and 3.9 mg of N,N′-methylenebisacrylamide were added and,furthermore, 0.05 g of ammonium persulfate was added and dissolved. Theresulting aqueous solution containing a polymerization initiator and acrosslinking agent, which is essentially composed of sodiumsulfoethylmethacrylate and acrylamide as a main component, was added tothe contents in a cylindrical round bottom flask and was dispersed in asiloxane solution containing a surfactant at a stirring rate of 300 rpmand, at the same time, the atmosphere in the system was sufficientlyreplaced by nitrogen. Then, the polymerization reaction was initiated byheating. After a while, heat was generated. After five minutes hadpassed since the peak of heat generation, the resulting dispersion of ahydrolysate of polysuccinimide having methacryloyl groups introducedtherein in an aqueous solution was added at a time. The mixture wasmaintained at 60 to 65° C. for three hours. After the completion of thereaction, the four-necked flask was equipped with a water dispenser andazeotropic dehydration was conducted by raising the temperature in thesystem to 70 to 80° C. After azeotropic dehydration was conducted untilthe amount of water in the system became 35% based on the solid contentto be charged, the temperature in the system was cooled to about 40° C.and a cyclohexane phase was separated by decantation. Subsequently,water was removed from a wet polymer by vacuum drying to obtain a waterabsorbent material as gel-like particles (hereinafter referred to as (3)second step). The evaluation results of this water absorbent materialare shown in Table 6.

Comparative Example 3

In a 500 ml four-necked flask equipped with a stirrer, a thermometer, areflux condenser, and a nitrogen gas introducing device, 164 g ofcyclohexane was added, and then 0.75 g of “DK ESTER F-90” was added anddissolved by heating to 50° C. with stirring. The contents in the flaskwere cooled to 30° C. In a 500 ml Erlenmeyer flask, 18.4 g of sodiumsulfoethylmethacrylate was added. To this solution, 18.4 g of acrylamideand 3.9 mg of N,N′-methylenebisacrylamide were added, and furthermore,0.05 g of ammonium persulfate was added and dissolved. The resultingaqueous solution containing a polymerization initiator and acrosslinking agent, which is essentially composed of sodiumsulfoethylmethacrylate and acrylamide as a main component, was added tothe contents in a cylindrical round bottom flask and was dispersed in asiloxane solution containing a surfactant at a stirring rate of 300 rpm,and at the same time, the atmosphere in the system was sufficientlyreplaced by nitrogen. Then, the polymerization reaction was initiated byheating. The mixture was maintained at 60 to 65° C. for three hours.After the completion of the reaction, the four-necked flask was equippedwith a water dispenser and azeotropic dehydration was conducted byraising the temperature in the system to 70 to 80° C. After azeotropicdehydration was conducted until the amount of water in the system became35% based on the solid content to be charged, the temperature in thesystem was cooled to about 40° C. and a cyclohexane phase was separatedby decantation. Subsequently, water was removed from a wet polymer byvacuum drying to obtain a water absorbent material as gel-like particles(hereinafter referred to as (3) third step). The evaluation results ofthis water absorbent material are shown in Table 6.

Example 7

In a 500 ml Erlenmeyer flask, 16.5 g of2-acrylamide-2-methylpropanesulfonic acid was charged and neutralized(60 mol %) by adding dropwise 78.4 g of an aqueous sodium hydroxidesolution containing 1.9 g of sodium hydroxide dissolved therein whilecooling from outside. To this solution, 14.7 g of acrylamide and 23.1 mgof N,N′-methylenebisacrylamide were added and then 0.05 g of ammoniumpersulfate was added and dissolved. The same operation was conducted,except that the resulting aqueous solution containing a polymerizationinitiator and a crosslinking agent, which is essentially composed of2-acrylamide-2-methylpropanesulfonic acid, a sodium salt thereof andacrylamide as a main component was used, a water absorbent material ofthe present invention as gel-like particles was obtained. The evaluationresults of characteristics of the water absorbent material are shown inTable 6.

Example 8

In a 500 ml Erlenmeyer flask, 30 g of polymer particles obtained inExample 7 were weighed, and then a mixed solution of 1.2 g of acetone,2.1 g of ion exchange water, 0.09 g of glycidyl methacrylate and 0.09 gof ammonium persulfate as well as 0.3 g of “200CF” [hydrophilic silica,manufactured by Nippon Aerosil Co., Ltd.] were uniformly dispersed,together with the polymer particles. The surface crosslinking treatmentof the polymer was conducted by vacuum-drying the wet polymer at 108° C.for one hour (hereinafter referred to as (3) third step). The evaluationresults of the resulting water absorbent material are shown in Table 6.

Comparative Example 4

The same operation as in Example 7 was conducted to prepare an aqueoussolution containing a polymerization initiator and a crosslinking agent,which is essentially composed of 2-acrylamide-2-methylpropanesulfonicacid, a sodium salt thereof and acrylamide as a main component, wasprepared. The same operation as in Comparative Example 3 was conductedto obtain a water absorbent material of the present invention asgel-like particles. The evaluation results of the resulting waterabsorbent material are shown in Table 6.

Comparative Example 5

4.44 g of polyethylene glycol diacrylate was dissolved in 5500 g of anaqueous solution of sodium acrylate (neutralization ratio: 75 mol %),and after deaerating with a nitrogen gas, 2.4 g of sodium persulfate and0.12 g of 1-ascorbic acid were added and the polymerization wasconducted. After the completion of the polymerization, the resultinghydrous gel-like particles were further ground and dried in a hot-airdryer at 150° C. so that the water content of the hydrous gel-likeparticles became 5% or less. The dried substance was granulated by aroll granulater and then passed through an ASTM 20 mesh metal wire toobtain a water absorbent polymer in an amorphous ground form.

100 parts of this water absorbent polymer was mixed with an aqueoussolution of 1 part of sodium polyaspartate (molecular weight: 10000) and5 parts of water to obtain an absorber composition. The evaluationresults of characteristics of this water absorbent material are shown inTable 6.

Components to be charged of Examples 6 to 8 and Comparative Examples 3and 4 are shown in Table 5.

TABLE 5 Comp. Comp. Components to be charged (g) Example 6 Example 7Example 8 Example 3 Example 4 (3) First step Polysuccinimide (2) 3 3 3 —— GMA 0.3 0.3 0.3 — NaOH 1.2 1.2 1.2 — — Ion exchange water 3.2 3.2 3.2— — Sucrose ester F-160 (HLB = 16) 0.75 0.75 0.75 — — Cyclohexane 20 2020 — 20 (3) Second step Sucrose ester F-90 (HLB = 9) 0.75 0.75 0.75 — —Cyclohexane 164 164 164 — — Na sulfomethyl methacrylate 18.4 — — 18.4 —AMPS — 16.5 16.5 16.5 — Acrylamide 18.4 18.4 18.4 18.4 18.4 NaOH — 1.91.9 — 8.3 Ion exchange water 80.9 76.5 76.5 80.9 76.5 MBAA 0.0039 0.00390.0039 0.0039 0.0039 APS 0.05 0.05 0.05 0.05 0.05 (3) Third step GMA — —0.09 — — APS — — 0.09 — — Ion exchange water — — 2.1 — — Acetone — — 1.2— — Hydrophilic silica 200CF — — 0.3 — — GMA: glycidyl methacrylateAMPS: 2-acrylamide-2-methylpropanesulfonic acid MBAA:N,N′-methylenebisacrylamide APS: ammonium persulfate Polysuccinimide(2): polysuccinimide (Mw: 3,000) obtained in Synthetic Example 2

TABLE 6 Water absorption ratio (g/g) 0.9% aqueous Water absorption Ionexchange water solution of NaCl Artificial seawater rate (seconds) Gelstrength (g/cm²) Example 6 715 121 55 120 2.4 Example 7 520 49 30 64 6.0Example 8 490 45 28 55 12.0 Comp. Example 3 587 98 47 195 2.4 Comp.Example 4 440 41 27 95 6.0 Comp. Example 5 380 37 3 257 1.2

As is apparent from Table 6, the water absorbent materials of thepresent invention have high water absorption ratio and high waterabsorption rate in ion exchange water, an aqueous 0.9% NaCl solution,and artificial seawater as compared with the water absorbent materialsof Comparative Examples and water absorption performances are improved.

INDUSTRIAL APPLICABILITY

The water absorbent material of the present invention has high waterabsorption properties for high concentration salt-containing solutionssuch as seawater, aqueous deliquescent calcium chloride solution, blood,and body fluids (e.g. urine, sweat, etc.) as compared with aconventional water absorbent material by using a copolymer containingpolysaccharides as a copolymer component, and can improve waterabsorption properties to salt-containing solutions without impairing awater absorption ratio and a water absorption rate by using a monomerhaving a sulfonic acid group containing ethylenically unsaturated doublebond. Therefore, the water absorbent material of the present inventioncan be widely used as water absorbent materials for water sealingmaterials, civil engineering materials, materials foragricultural/horticultural disposable sanitary materials, and householdarticles.

The absorbent article of the present invention is suited for use indisposable diapers for infants, adults, and persons suffering fromincontinence, and in sanitary napkins because of its excellentabsorbency of urine and body fluids as well as excellent urine leakageinhibition effects.

1. A water absorbent material comprising a copolymer of ananhydropolyamino acid having at least one ethylenically unsaturateddouble bond in a molecule (A) and a water-soluble monomer having anethylenically unsaturated double bond (B).
 2. A water absorbent materialaccording to claim 1, wherein the water-soluble monomer having anethylenically unsaturated double bond (B) is at least one selected fromthe group consisting of (meth)acrylic acid, alkali metal salt of(meth)acrylic acid, ammonium salt of (meth)acrylic acid, and an amidatedcompound of (meth)acrylic acid.
 3. A water absorbent material accordingto claim 1, wherein the water-soluble monomer having an ethylenicallyunsaturated double bond (B) is a monomer having an ethylenicallyunsaturated double bond, and a sulfonic acid group and/or a sulfonategroup in a molecule.
 4. A water absorbent material according to claim 1,wherein the anhydropolyamino acid having at least one ethylenicallyunsaturated double bond in a molecule (A) is a reaction product of ananhydropolyamino acid having no ethylenically unsaturated double bond ina molecule (A-1) and a compound which has an ethylenically unsaturateddouble bond and a functional group having reactivity with theanhydropolyamino acid (A-1) in a molecule (A-2).
 5. A water absorbentmaterial according to claim 1, wherein the copolymer comprises gelparticles.
 6. A water absorbent material according to claim 4, whereinthe compound which has an ethylenically unsaturated double bond and afunctional group having reactivity with the anhydropolyamino acid (A-1)in a molecule (A-2) is a compound represented by the following generalformula [I]:

wherein R¹ represents at least one functional group selected from thegroup consisting of amino group, epoxy group, carboxyl group,carbodimide group, oxazoline group, imino group and isocyanate group, Qrepresents an alkylene group having 1 to 10 carbon atoms, and R²represents hydrogen or an alkylene group having 1 to 4 carbon atoms. 7.A water absorbent material according to claim 4, wherein theanhydropolyamino acid having no ethylenically unsaturated double bond ina molecule (A-1) is polysuccinimide.
 8. A water absorbent materialaccording to claim 1, wherein a portion or all of the anhydropolyaminoacid having an ethylenically unsaturated double bond in a molecule (A)is hydrolyzed.
 9. A water absorbent material according to claim 1,wherein a water absorption ratio of a physiological saline solution is10 g/g or more.
 10. An absorbent article comprising an absorbercomprising a water absorbent material and a fiber material arrangedbetween a liquid-permeable sheet and a liquid-impermeable sheet, whereinthe water absorbent material is a water absorbent material comprising acopolymer of an anhydropolyamino acid having at least one ethylenicallyunsaturated double bond in a molecule (A) and a water-soluble monomerhaving an ethylenically unsaturated double bond (B).
 11. An absorbentarticle according to claim 10, wherein the copolymer comprises gelparticles.